1. Field of the Invention
This invention relates to an electronic picture display system wherein discrete image defining signals control a raster scan display screen In particular the invention relates to a display system in which image pel definition signals are stored in a data processor system along with a list of operations which are to be applied to the image pel definition signals prior to them being displayed Both the image pel definitions and the list of operations are transmitted in response to control signals to a raster scan display device containing appropriate data processing capability to perform the list of operations to the image pel definitions, and to display the resulting image on the display surface.
2. Description of the Related Art
In this description the term `image` is used to mean a picture that is defined by a series of discrete picture elements (pels) and can be displayed on a display screen such as a raster driven cathode ray tube. This use is to distinguish from a graphic picture which is made up of lines and areas and is stored as signals that define the coordinate positions of line endings and the background of areas.
A pel may vary in size and density depending upon the characteristics of the display device.
The magnification or scale of an image is changed by varying the number of pels used to display the whole or part of the image.
A transform operation is an operation on the whole or a selected portion of an image to change, for example, its scale, its orientation or its position relative to the rest of the screen.
There are today output devices which support image datastream containing image transform operations such as Extract rectangle, Scale rectangle, Orientate rectangle, Reflect rectangle and Place rectangle, which are transmitted to the display device before transmitting the image on which they are to be performed. The display device then performs the operations on each row or raster line of image data as they are received, (i.e. row by row from top to bottom of the image), applying the required transform operations on it, and placing it on the display surface at the positions defined by the Place rectangle operations. Image data which does not fall inside an Extract rectangle is discarded. This approach means that the device does not require storage in which to buffer the entire image data signals, prior to applying the requested transform operations, hence the production cost of each device is significantly reduced.
One of two problems may arise however, when two or more transformed rectangles are placed such that they overlap on the display surface with the overlapping portions being combined using a mixing or merging algorithm which is not commutative. A commutative mixing algorithm is one which produces the same result when image A overlaps image B as when image B overlaps image A. An example of a non-commutative mixing algorithm is "OVERPAINT", if image A Overpaints a portion of image B them that portion of image B is completely obscured.
The first problem is caused by transformed rectangles being placed on the screen in the order that the row or raster lines which define the Extract rectangles are positioned in the incoming image data signals, (i.e. from top to bottom of the source image), rather than in the order they are defined by the transform operations. This results for instance, in image B being overwritten by image A, instead of A being overwritten by B.
The second related problem occurs when Extract rectangles overlap vertically in the incoming source image, and their transformed rectangles are to be placed such that they overlap on the display surface. This results in an interference pattern appearing within the overlapping area, as the transformed rectangles are written onto the screen, with successive rows from each of the rectangles in turn being placed on the screen so that they overlap rows which have previously been placed and which form part of the other rectangles.